Re: [PATCH v8 9/9] manual: Add documentation for arc4random functions

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Wed, Jun 29, 2022 at 2:36 PM Adhemerval Zanella via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> ---
>  manual/math.texi | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/manual/math.texi b/manual/math.texi
> index 477a18b6d1..ab96726e57 100644
> --- a/manual/math.texi
> +++ b/manual/math.texi
> @@ -1447,6 +1447,7 @@ systems.
>  * ISO Random::                  @code{rand} and friends.
>  * BSD Random::                  @code{random} and friends.
>  * SVID Random::                 @code{drand48} and friends.
> +* High Quality Random::         @code{arc4random} and friends.
>  @end menu
>
>  @node ISO Random
> @@ -1985,6 +1986,50 @@ This function is a GNU extension and should not be used in portable
>  programs.
>  @end deftypefun
>
> +@node High Quality Random
> +@subsection High Quality Random Number Functions
> +
> +This section describes the random number functions provided as a GNU
> +extension, based on OpenBSD interfaces.
> +
> +@Theglibc{} uses kernel entropy obtained either through @code{getrandom}
> +or by reading @file{/dev/urandom} to seed and periodically re-seed the
> +internal state.  A per-thread data pool is used, which allows fast output
> +generation.
> +

Are we committing to per-thread data pools? I thought there were ideas to
use rseq.


> +Although these functions provide higher random quality than ISO, BSD, and
> +SVID functions, these still use a Pseudo-Random generator and should not
> +be used in cryptographic contexts.
> +
> +The internal state is cleared and reseed with kernel entropy on @code{fork}
> +and @code{_Fork}.  It is not cleared for either direct @code{clone} syscall
> +or when using @theglibc{} @code{syscall} function.
> +
> +The prototypes for these functions are in @file{stdlib.h}.
> +@pindex stdlib.h
> +
> +@deftypefun int32_t arc4random (void)
> +@standards{BSD, stdlib.h}
> +@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
> +This function returns a single 32-bit value in the range of @code{0} to
> +@code{2^32−1} (inclusive), which is twice the range of @code{rand} and
> +@code{random}.
> +@end deftypefun
> +
> +@deftypefun void arc4random (void *@var{buffer}, size_t @var{length})
> +@standards{BSD, stdlib.h}
> +@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
> +This function fills the region @var{buffer} of @var{length} with random data.
> +@end deftypefun
> +
> +@deftypefun uint32_t arc4random_uniform (uint32_t @var{upper_bound})
> +@standards{BSD, stdlib.h}
> +@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acsafe{}}
> +This function returns a single 32-bit value, uniformly distributed but
> +less than the @var{upper_bound}.  It avoids the @w{modulo bias} when the
> +upper bound is not a power of two.
> +@end deftypefun
> +
>  @node FP Function Optimizations
>  @section Is Fast Code or Small Code preferred?
>  @cindex Optimization
> --
> 2.34.1
>